Buried heterostructure device fabricated by single step MOCVD

ABSTRACT

The device is an optoelectronic device or transparent waveguide device that comprises a growth surface, a growth mask, an optical waveguide core mesa and a cladding layer. The growth mask is located on the semiconductor surface and defines an elongate growth window. The optical waveguide core mesa is located in the growth window and has a trapezoidal cross-sectional shape. The cladding layer covers the optical waveguide core mesa and extends over at least part of the growth mask. Such devices are fabricated by providing a wafer comprising a growth surface, growing an optical waveguide core mesa on the growth surface by micro-selective area growth at a first growth temperature and covering the optical waveguide core mesa with cladding material at a second growth temperature, lower than the first growth temperature.

BACKGROUND

Optoelectronic devices are used in many applications includingtelecommunications, data storage and signalling. Certain types ofoptoelectronic devices such as laser diodes, optoelectronic modulators,semiconductor optical amplifiers, semiconductor gain media, etc., havean active region located in an optical waveguide. The optical waveguidetypically incorporates different structures to guide the lightlaterally, i.e., parallel to the major surface of the substrate on whichthe device is fabricated, and transversely, i.e., orthogonal to themajor surface of the substrate. In the transverse direction, the lightis guided by a refractive index contrast between the semiconductormaterial of the active region and cladding layers between which theactive layer is sandwiched. In the lateral direction, the light isguided by a ridge waveguide structure or a buried heterostructurewaveguide defined at least in part in the layer structure of which thecladding layers and the active region form part.

In telecommunications applications, the most commonly used lateralwaveguide structure is the buried heterostructure. A buriedheterostructure provides advantages over a ridge waveguide structurebecause of the large refractive index contrast it provides at the activeregion. This allows the optical waveguide to be made very narrow, whilepreserving a high spatial overlap between the fundamental optical modeand the active region. This provides such advantages as a low thresholdcurrent in lasers, a lower operating current in semiconductor opticalamplifiers and optical gain media, and a low capacitance, and, hence,increased modulation speed, in optoelectronic modulators and directlymodulated lasers.

A typical process for fabricating optoelectronic devices incorporating aburied heterostructure lateral waveguide is illustrated in FIGS. 1A–1C.First, a layer structure 10 from which hundreds or thousands ofoptoelectronic devices are made is grown. FIGS. 1A–1C are side views ofa portion of layer structure 10 in which a single optoelectronic deviceis fabricated. FIG. 1A shows an n-type cladding layer 12, an undopedactive region 14 and a p-type cladding layer 16 grown on a substrate 18.The layers are grown by metal organic chemical vapor deposition (MOCVD),also known in the art as organo-metallic vapor phase epitaxy (OMVPE).

The materials of layer structure 10 are Group III–V semiconductorstypically composed of such elements as indium, gallium, arsenic andphosphorus. The semiconductor material of cladding layers 12 and 16 hasa lower refractive index than that of active region 14. The thickness ofn-type cladding layer 12 is about 2 μm, whereas the thickness of p-typecladding layer 16 in layer structure 10 is only about 200 nm–400 nm.

A quantum well structure 20 composed of one or more quantum wells islocated in active region 14. Each quantum well is defined by a quantumwell layer of low band-gap semiconductor material sandwiched betweenbarrier layers of higher band-gap semiconductor material.

FIG. 1A also shows a mask 22 deposited on the surface of p-type claddinglayer 16. The material of the mask is typically silicon dioxide. Mask 22is elongate in the y-direction shown in FIG. 1A, and is typically about1–8 μm wide.

Layer structure 10 is then removed from the growth chamber and issubject to two etching processes that define a mesa 24 in the layerstructure, as shown in FIG. 1B. A reactive ion etch (RIE) is initiallyused to remove portions of p-type cladding layer 16, active region 14and n-type cladding layer 12 not protected by mask 22. The RIE damagesthe edges of the layers subject to etching. Such damaged edgessignificantly impair the efficiency of the finished optoelectronicdevice. Accordingly, layer structure 10 is additionally subject to a wetetch that removes the damaged edges of p-type cladding layer 16, activeregion 14 and n-type cladding layer 12. The wet etch processadditionally defines the overhang of mask 22 relative to mesa 24. FIG.1B shows layer structure 10 after both etching processes have beenperformed.

Layer structure 10 is then returned to the growth chamber, and anovergrowth 26 of a high resistivity group III–V semiconductor materialhaving a lower refractive index than the materials of active region 14is epitaxially grown on the layer structure by MOCVD, as shown in FIG.1C. The overgrowth grows on the exposed surface of substrate 18 and onthe sidewalls of mesa 24, but does not grow on mask 22. Accordingly,overgrowth 26 fills the cavities etched into the layer structure betweenadjacent mesas. Deposition of the overgrowth continues until its growthsurface reaches the top surface of p-type cladding layer 16.

In an embodiment of layer structure 10 in which the material of claddinglayers 12 and 16 is indium phosphide (InP), a typical material ofovergrowth 26 is indium phosphide doped with iron (InP:Fe). Therefractive index of the overgrowth material is about 0.2 less than thatof the materials of active region 14. The overgrowth material is dopedwith iron (Fe) to reduce its conductivity.

Layer structure 10 is then removed from the growth chamber and issubject to another wet etch process that removes the mask 22 from thesurface of p-type cladding layer 16.

Layer structure 10 is then returned to the growth chamber, whereadditional p-type cladding layer material 28 is grown over the exposedsurfaces of p-type cladding layer 16 and overgrowth 26, as shown in FIG.1C. P-type cladding layer 16 and the portion of the additional p-typecladding layer material grown on p-type cladding layer 16 collectivelyconstitute p-type cladding layer 30. P-type cladding layer typically hasa thickness about the same as that of n-type cladding layer 12, i.e.,about 2 μm.

A p-contact layer (not shown) is grown on top of p-type cladding layer30, and electrode layers (not shown) are deposited on the bottom surfaceof substrate 18 and the exposed surface of the p-contact layer. Theelectrode layers are then patterned to define electrodes. Layerstructure 10 is then singulated into individual optoelectronic devices.

Although the above-described buried heterostructure waveguide providesperformance advantages, the above-described fabrication process iscomplex and is difficult to control. In particular, it is essential toetch the layer structure using a low-damage etch process, since the etchproceeds through the p-i-n junction formed by layers of p-type, undopedand n-type material (not shown) in active region 14. It is highlyundesirable to have carrier states associated with structural defects inthe etched sidewalls of the mesa. Moreover, the width of active region14, i.e., the dimension of the active region in the x-direction shown inFIG. 1A, is defined by the etch process. The width of the active regionhas to be accurately defined: too narrow an active region results ininsufficient gain or too high a threshold current. Too wide an activeregion allows the optoelectronic device to operate in multiple opticalmodes, which is undesirable in many applications. Finally, the undercutprofile of mesa 24 relative to mask 22 must also be accuratelycontrolled to ensure that overgrowth 26 provides a reasonably planarsurface on which to grow the additional p-type cladding layer material28.

Optoelectronic devices for use in long wavelength telecommunicationsapplications originally had indium gallium arsenide phosphide (InGaAsP)as the material of the quantum well layers. Using aluminum indiumgallium arsenide (AlInGaAs) instead of InGaAsP as the material of thequantum well layers improves the high temperature characteristics of theoptoelectronic device. However, using AlInGaAs as the material of thequantum well layers makes fabrication of the buried heterostructurewaveguide structure considerably more difficult. This is because thepresence of aluminum in the material of the quantum well layers leads tothe formation of a stable layer of oxide on the sidewall of the mesa 24during the wet etch. Unlike the less-stable oxides of indium and galliumformed when InGaAsP is etched, aluminum oxide cannot be thermallydesorbed in the MOCVD growth chamber prior to growing overgrowth 26.Instead, the aluminum oxide layer persists on the sidewalls of the mesa,and degrades the quality of the interface between the mesa andovergrowth 26.

The problem of damage to the exposed sidewalls of the mesa 24 isexacerbated by the need to transfer the wafer from the etch station tothe growth chamber after the etch process has been performed.Transferring the mesa 24 from the etch station to the growth chamberexposes the sidewalls of the mesa 24 to ambient air, which typicallycontains water vapor and oxygen. The water vapor and oxygen can causeadditional oxide formation on the sidewalls of the mesa 24.

Various approaches have been proposed to deal with the problem of stablealuminum oxides forming on the sidewalls of the mesa. For example,in-situ etching may be used, as described by Bertone et al. in Etchingof InP-based MQW Structure in a MOCVD Reactor by Chlorinated Compounds,195 J. CRYST. GROWTH, 624 (1998). However, such approaches are expensiveand difficult to implement, and may be incompatible with fabricationprocesses for other devices.

In Densely Arrayed Eight-Wavelength Semiconductor Lasers Fabricated byMicroarray Selective Epitaxy, 5 IEEE J. SEL. Top. QUANTUM ELECTRON., 428(1999), K. Kudo et al. disclose a process for fabricating an array ofburied heterostructure lasers using micro-selective area growth. Thisprocess is illustrated in FIGS. 2A–2C. FIG. 2A shows a substrate 68 onwhich an n-type cladding layer 62 has been grown. An optical waveguidecore mesa 80 that additionally constitutes the active region 64 of theoptoelectronic device is then grown by micro-selective area growth onthe surface of n-type cladding layer 62. The optical waveguide core mesais grown in an elongate window 82 defined by two elongate mask patterns84. The optical waveguide core mesa has a trapezoidal cross-sectionalshape, and is elongate in the y-direction shown.

Using micro-selective area growth to fabricate an optical waveguide coremesa that includes the active region of a buried heterostructure laserimproves the dimensional accuracy of the active region. Additionally,using micro-selective area growth forms the optical waveguide core mesawithout the need to etch through the active region. However, a secondmicro-selective area growth process is used to cover optical waveguidecore mesa 80 with p-type cladding material. The second micro-selectivearea growth process involves removing the wafer from the growth chamberand etching mask patterns 84 to increase the width of window 82. FIG. 2Bshows narrowed mask patterns 86 and widened window 88 resulting frometching the mask patterns 84 shown in FIG. 2A.

The wafer is then returned to the growth chamber and a p-type claddingmesa 90 is grown over optical waveguide core mesa 80, as shown in FIG.2C. Cladding mesa 90 is grown by micro-selective area growth in thewidened window 88 defined by narrowed mask patterns 86 on the surface ofn-type cladding layer 62. Cladding mesa 90 has a trapezoidalcross-sectional shape and covers the sidewalls and top surface ofoptical waveguide core mesa 80.

Accordingly, although using micro-selective area growth to fabricate aburied heterostructure optoelectronic device obviates the need to etchthrough the active region itself, using a micro-selective area growthprocess that involves an intervening etch process, as disclosed by Kudoet al., does not provide a complete solution to the problems describedabove. The need to remove the wafer from the growth chamber to etch themask patterns exposes the sidewalls of the optical waveguide core mesato ambient air and, hence, to the possibility of stable oxide formationor other damage to the sidewalls. Additionally, the sidewalls of theoptical waveguide core mesa are exposed to the etchant used to etch themask patterns. This can result in stable oxide formation on, or otherdamage to, the sidewalls of the optical waveguide core mesa, especiallywhen the quantum well structure contains aluminum. The devices disclosedby Kudo et al. had quantum well layers of InGaAsP.

Moreover, the optoelectronic devices fabricated by the process disclosedby Kudo et al. have a high inter-electrode capacitance because anappreciable area of cladding mesa 90 abuts n-type cladding layer 62.Finally, cladding mesa 90 has a relatively narrow top surface on whichit is difficult to form the p-contact electrode.

Thus, what is needed is a way of fabricating optical waveguides andoptoelectronic devices incorporating a buried heterostructure lateralwaveguide structure that does not have the disadvantages of the buriedheterostructure fabrication processes described above. What is alsoneeded is a way of fabricating buried heterostructure optical waveguidesand optoelectronic devices whose optical waveguide cores includealuminum. Finally, what is needed are optical waveguides andoptoelectronic devices incorporating a buried heterostructure lateralwaveguide structure that does not have the disadvantages of the buriedheterostructure lateral waveguide structures described above.

SUMMARY

The invention provides in a first aspect an optical waveguide oroptoelectronic device that comprises a growth surface, a growth mask, anoptical waveguide core mesa and a cladding layer. The growth mask islocated on the growth surface and defines an elongate growth window. Theoptical waveguide core mesa is located in the growth window and has atrapezoidal cross-sectional shape. The cladding layer covers the opticalwaveguide core mesa and extends over at least part of the growth mask.

The invention provides in a second aspect a device fabrication method inwhich a growth chamber is provided, a wafer having a growth surface isprovided and a fabrication process is performed in the growth chamber.The fabrication process comprises growing an optical waveguide core mesaon the growth surface by micro-selective area growth, and, withoutremoving the wafer from the growth chamber after fabricating the opticalwaveguide core mesa, covering the optical waveguide core mesa withcladding material.

The cladding material is grown without removing the substrate from thegrowth chamber by growing the cladding material under growth conditionsin which the cladding layer material grows on the sidewalls of theoptical waveguide core mesa in addition to the top surface of the mesawithout the need to perform an etch process prior to growing thecladding material. The cladding material covering the optical waveguidecore mesa protects the sidewalls of the optical waveguide core mesa frometchants and atmospheric contamination when the device is eventuallyremoved from the growth chamber for further processing.

The invention provides in a third aspect a device fabrication method inwhich a wafer having a growth surface is provided, an optical waveguidecore mesa is grown on the growth surface by micro-selective area growthat a first growth temperature and the optical waveguide core mesa iscovered with cladding material at a second growth temperature, lowerthan the first growth temperature.

Covering the optical waveguide core mesa with the cladding material atthe lower growth temperature allows the cladding material to grow on thesidewalls of the optical waveguide core mesa without the need to performan etch process prior to growing the cladding material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A–1C illustrate the fabrication of a first type of prior artoptoelectronic device incorporating a buried heterostructure opticalwaveguide.

FIGS. 2A–2C illustrate the fabrication of a second type of prior artoptoelectronic device incorporating a buried heterostructure opticalwaveguide.

FIGS. 3A–3G illustrate the fabrication of an optoelectronic deviceincorporating a buried heterostructure optical waveguide in accordancewith an embodiment of the invention.

FIG. 3H is an enlarged view of part of FIG. 3F showing one sidewall ofthe optical waveguide core mesa.

FIG. 4A is an isometric view of an exemplary embodiment of anoptoelectronic device incorporating a buried heterostructure opticalwaveguide in accordance with the invention.

FIG. 4B is an enlarged view of showing the structure of the opticalwaveguide core mesa of the exemplary optoelectronic device shown in FIG.4A.

FIGS. 5A and 5B are graphs showing some performance characteristics ofan embodiment of an optoelectronic device in accordance with theinvention configured as a buried heterostructure laser.

DETAILED DESCRIPTION

The invention is based on the realization that the problems arising fromexposing an optical waveguide core mesa grown by micro-selective areagrowth to an etchant and/or the atmosphere can be avoided by notwidening the mask window before growing the p-type cladding layer.Instead, in accordance with the invention, the p-type cladding layer isgrown under growth conditions that cause the p-type cladding layermaterial to grow on the sidewalls of the optical waveguide core mesa inaddition to the top surface thereof. In accordance with an embodiment ofthe invention, after a thin layer of p-type cladding layer material hasbeen grown on the top surface of the optical waveguide core mesa, thegrowth temperature is reduced to one at which the surface migrationlength of the p-type cladding material is less than the width of thesidewalls of the mesa. Under these growth conditions, the p-typecladding layer material grows not only on the top surface of the opticalwaveguide core mesa, but also on the sidewalls. This way, a p-typecladding layer that covers the sidewalls of the optical waveguide coremesa is grown before the wafer is exposed to an etchant and/or to theatmosphere or is subject to conditions that could otherwise damage thesidewalls of the optical waveguide core mesa.

FIGS. 3A–3G illustrate an exemplary embodiment of a method in accordancewith the invention for making a device. In the example shown, anoptoelectronic device is fabricated. Hundreds or thousands ofoptoelectronic devices are made at a time on a single wafer. The waferis then singulated to yield the individual optoelectronic devices. Otherembodiments of the method fabricate transparent waveguide devices, suchas buried heterostructure optical waveguides.

FIG. 3A is a side view of the small portion of a wafer 110 (FIG. 3D)that constitutes the substrate 112 of an exemplary one of theoptoelectronic devices fabricated on the wafer. The crystallineorientation on the major surface 114 of wafer 110 is [100]. In anexemplary embodiment, the material of the wafer is n-type indiumphosphide (InP).

Wafer 110 is mounted on the susceptor (not shown) of a metal-organicchemical vapor deposition (MOCVD) growth chamber (not shown) and ann-type cladding layer 120 of n-type indium phosphide is grown on majorsurface 114, as shown in FIG. 3B. The n-type cladding layer is grown toa thickness of about 2 μm. The exposed major surface of n-type claddinglayer 120 provides a growth surface 122 on which an optical waveguidecore mesa is grown.

A mask layer (not shown) is then deposited on growth surface 122. In anembodiment, the material of the mask layer was silicon dioxide (SiO₂).Wafer 110 is then removed from the growth chamber, and is subject tophotolithography and etching to pattern the mask layer to define agrowth mask 130, as shown in FIG. 3C.

FIG. 3D is a plan view of wafer 110 showing growth masks 130 arrayed onthe growth surface 122 of n-type cladding layer 120 (FIG. 3C) grown onthe wafer. Each growth mask is composed of a pair of mask stripes 132.FIG. 3D is highly simplified in the sense that it shows only threegrowth masks. In a practical embodiment, each growth mask 130 is about10–25 μm wide, and adjacent growth masks are separated by a distance inthe range from about 100 μm to about 500 μm across the width of thewafer, so a typical wafer has hundreds of growth masks arrayed on itssurface. A flat 116 indicates the orientation of the [0-1-1] normal tothe crystalline plane of the wafer.

Mask stripes 132 are elongate and have their long sides aligned parallelto the [011] crystalline direction of growth surface 122, which isaligned in the y-direction shown. Each pair of adjacent ones of maskstripes 132 constitutes a growth mask 130 that defines an elongategrowth window 134 in which an elongate waveguide core mesa will be grownby micro-selective area growth. The width of the growth window, definedby the distance in the x-direction between the opposed edges of the pairof the mask stripes 132, is in the range from about 1 μm and about 3 μm,and is typically in the range from about 1.5 μm to about 2 μm. Theactual width of growth window 134 depends on the specified width of thequantum well structure of the optoelectronic device being made, i.e.,the specified dimension in the x-direction of the quantum wellstructure.

Wafer 110 is returned to the MOCVD growth chamber, and optical waveguidecore mesa 140 is grown in growth window 134 using micro-selective areagrowth. In the example of the optoelectronic device shown, opticalwaveguide core mesa 140 is structured to provide the active region ofthe optoelectronic device, and is composed of, in order from growthsurface 122, an n-type buffer layer, a hole blocking layer, asubstrate-side separation layer, a quantum well structure, a remote-sideseparation layer and an electron blocking layer. The structure of theoptical waveguide core mesa will be described in detail below withreference to FIG. 4B. At least one quantum well, composed of a quantumwell layer (also shown in FIG. 4B) sandwiched between two barrier layers(also shown in FIG. 4B) is located in the quantum well region. In atransparent waveguide device, optical waveguide core mesa 140 ishomogeneous and lacks the layers shown in FIG. 4B.

During the growth of optical waveguide core mesa 140 by micro-selectivearea growth, semiconductor material formed from the precursors fed tothe MOCVD growth chamber alights on growth mask 130. This semiconductormaterial does not nucleate on the growth mask, but migrates towards theportion of growth surface 122 exposed in growth window 134. Thesemiconductor material growing in the growth window has a strongtendency to form a [111] sidewall on which the growth rate isapproximately zero. Accordingly, the semiconductor material growspredominantly on the top surface 146 of optical waveguide core mesa 140,and the optical waveguide core mesa grows in growth window 134 with thetrapezoidal cross-sectional shape shown in FIG. 3E. The opticalwaveguide core mesa is bounded by straight, smooth, [111] sidewalls 144.

Growth of optical waveguide core mesa 140 is performed at a growthtemperature at which the adatoms of the semiconductor material havesufficient mobility that their surface diffusion length is greater thanthe width w (FIG. 3H) of the [111] surfaces constituting sidewalls 144.As long as the surface diffusion length is greater than the width ofsidewalls 144, substantially no semiconductor material grows on thesidewalls.

Optical waveguide core mesa 140 is grown until it reaches its specifiedthickness. Then, the precursors fed to the growth chamber are changed tothose for the p-type cladding layer material and the growth temperatureis reduced slightly relative to that used to grow the optical waveguidecore mesa. However, the reduced temperature is still above thetemperature at which the surface diffusion length of the adatoms of thesemiconductor material is greater than the width of sidewalls 144.Consequently, a thin sublayer 162 of the p-type cladding layer grows onthe top surface of optical waveguide core mesa 140, as shown in FIG. 3F.

After sublayer 162 of the p-type cladding layer has reached a thicknessof a few tens of nanometers, the growth temperature is reduced to one atwhich the mobility of the adatoms of semiconductor material is such thattheir surface diffusion length is less than the width of sidewalls 144.Micro-selective area growth continues at the reduced growth temperature,and the semiconductor material continues not to nucleate on growth mask130. However, at the reduced growth temperature, growth no longerpredominantly occurs on the top surface 146 of optical waveguide coremesa 140. Consequently, the remainder of p-type cladding layer 160 growson the sidewalls 144 of optical waveguide core mesa 140 in addition togrowing on top surface 146.

Growth of p-type cladding layer 160 at the reduced growth temperature iscontinued until the p-type cladding layer reaches its specifiedthickness, as shown in FIG. 3G. As the p-type cladding layer grows onsidewalls 144, it additionally extends laterally over part of growthmask 130. The p-type cladding layer forms a plane major surface 164 towhich an electrode can later be applied.

After growth of p-type cladding layer 160 has been completed, wafer 110is removed from the growth chamber. P-type cladding layer 160 covers thesidewalls 144 of optical waveguide core mesa 140 and therefore protectsthe sidewalls from the environment. Accordingly, the p-type claddinglayer protects the sidewalls from damage during subsequent processingapplied to the wafer, such as electrode application, electrodepatterning and singulation.

FIG. 4A is an isometric view of an exemplary embodiment of anoptoelectronic device 100 in accordance with the invention fabricated bythe above-described fabrication method in accordance with the invention.FIG. 4A does not show the layer structure of the optical waveguide coremesa of optoelectronic device 100 to simplify the drawing. FIG. 4B is anenlarged view of part of optoelectronic device 100 showing the layerstructure of the optical waveguide core mesa.

Referring first to FIG. 4A, optoelectronic device 100 is composed of agrowth surface 122, a growth mask 130, an optical waveguide core mesa140 and a cladding layer 160. Growth mask 130 is located on growthsurface 122 and defines an elongate growth window 134. Optical waveguidecore mesa 140 is located in the growth window and has a trapezoidalcross-sectional shape. Cladding layer 160 covers optical waveguide core140 and at least part of growth mask 130.

In the example shown, growth surface 122 is the major surface of ann-type cladding layer 120 epitaxially grown on a substrate 112. In anembodiment, the material of substrate 112 is single-crystal n-typeindium phosphide (InP), n-type cladding layer 120 is a layer of n-typeInP and has a thickness of about 2 μm, and growth surface 122 has a[100] crystalline orientation.

In the example shown, growth mask 130 is composed of elongate,rectangular mask stripes 132. Mask stripes 132 are regions of silicondioxide (SiO₂) having opposed parallel edges that define elongate growthwindow 134. Growth window 134 has a width in the range from about 1 μmto about 3 μm, and typically in the range from about 1.5 μm to about 2μm. The actual width of the growth window is determined by the specifiedwidth of the quantum well region (154 in FIG. 4B), the distance betweengrowth surface 122 and the quantum well region, and the angle betweensidewalls 144 and growth surface 122. Mask stripes 132 each have a widthin the range from about 3 μm to about 11 μm. In the example shown, maskstripes 132 have a thickness of about 500 nm, which is similar (±150 nm)to the height of optical waveguide core mesa 140. The opposed edges ofthe growth stripes are aligned parallel to the [011] crystallinedirection of growth surface 122.

An alternative material of growth mask 130 is silicon nitride Si₃N₄.

Optical waveguide core mesa 140 is located on the growth surface 122 ofn-type cladding layer 120 in growth window 134 defined by growth mask130. Optical waveguide core mesa is composed of one or more layers ofone or more semiconductor materials having a higher refractive indexthan either of n-type cladding layer 120 and p-type cladding layer 160.In an embodiment, the refractive index contrast between opticalwaveguide core mesa 140 and cladding layers 120 and 160 was about −0.2.Optical waveguide core mesa 140 has a trapezoidal cross-sectional shapeas a result of its fabrication by micro-selective area growth that forms[111] surfaces that constitute its sidewalls 144.

P-type cladding layer 160 covers optical waveguide core mesa 140 and atleast part of growth mask 130. In particular, cladding layer 160contacts sidewalls 144 of optical waveguide core mesa 140. In theexample shown, the material of cladding layer 160 is p-type InP. Opticalwaveguide core mesa 140 is thus surrounded by n-type cladding layer 120and p-type cladding layer 160, which have a greater refractive indexthan the materials of the optical waveguide core mesa. Thus, opticalwaveguide core mesa 140 and cladding layers 120 and 160 collectivelyconstitute an optical waveguide.

In the example shown, optical waveguide core mesa 140 is structured toprovide the active region of optoelectronic device 100. FIG. 4B showsthe structure of an example of such an optical waveguide core mesa 140composed of, in order, an n-type buffer layer 151, a hole blocking layer152, a substrate-side separation layer 153, a quantum well structure154, a remote-side separation layer 155 and an electron blocking layer156. N-type buffer layer 151 is located on the growth surface 122 ofn-type cladding layer 120. Blocking layers 152 and 156 are layers ofsemiconductor materials significantly higher in band gap energy than thesemiconductor materials of separation layer 153 and 155. The structurecomposed of hole blocking layer 152, separation layers 153 and 155 andelectron blocking layer 156 forms a separate confinement heterostructure(SCH) 159 that confines current carriers (i.e., electrons and holes) toquantum well structure 154.

N-type buffer layer 151 is a layer of n-type InP about 100 nm thickgrown in growth window 134 on growth surface 122 of n-type claddinglayer 120.

Hole blocking layer 152 is a layer of n-type semiconductor materialhaving a band gap energy higher than the materials of the adjacentlayers, i.e., n-type buffer layer 151 and substrate-side separationlayer 153. In an embodiment, hole blocking layer 152 was a layer ofn-type aluminum indium arsenide (AlInAs) about 40 nm thick.

Substrate-side separation layer 153 is a layer of a semiconductormaterial having a band gap energy similar to that of the barrier layersof quantum well structure 154. No dopant was added to the material ofthe substrate-side separation layer during growth. In an embodiment,substrate-side separation layer 153 was a layer of AlGaInAs with Al, Gaand In fractions of 0.325, 0.175 and 0.5, respectively, about 50 nmthick.

Quantum well structure 154 is composed of N quantum well layers 157interleaved with N+1 barrier layers 158, where N is a positive integer.In the example shown, N=7. The material of the quantum well layers has asubstantially lower band gap energy that that of the barrier layers. Nodopant is added to the materials of the quantum well structure duringgrowth. In an embodiment, quantum well structure 154 was composed ofseven quantum well layers 157, each about 9 nm thick, and eight barrierlayers 158, each about 8 nm thick. The material of quantum wellstructure 154 was AlGaInAs with Al, Ga and In fractions of 0.18, 0.22and 0.6, respectively, in quantum well layers 157 and 0.32, 0.22 and0.46, respectively, in barrier layers 158.

Remote-side separation layer 155 is a layer of a semiconductor materialhaving a band gap energy similar to that of barrier layers 158 ofquantum well structure 154. No dopant is added to the material of theremote-side separation layer during growth. In an embodiment,remote-side separation layer 155 was a layer of AlGaInAs with Al, Ga andIn fractions of 0.325, 0.175 and 0.5, respectively, about 50 nm thick.

Electron blocking layer 156 is a layer of p-type semiconductor materialhaving a band gap energy higher than that of the materials of theadjacent layers, i.e., remote-side separation layer 155 and p-typecladding layer 160. In an embodiment, electron blocking layer 156 was alayer of p-type aluminum indium arsenide (AlInAs) about 40 nm thick.

Referring again to FIG. 4A, optoelectronic device 100 additionally hasan electrode 172 located on the surface of substrate 112 remote fromn-type cladding layer 120, an electrode 174 located on the surface 164of p-type cladding layer 160, and opposed facets 176 and 178 disposedorthogonally to the long axis of optical waveguide core mesa 140. Facets176 and 178 are typically formed by cleaving. In an embodiment ofoptoelectronic device 100 in which facets 176 and 178 are highlyreflective, current flowing between electrodes 174 and 172 causesoptoelectronic device 100 to operate as a laser and generate coherentlight that is emitted through the facets. In an embodiment ofoptoelectronic device 100 in which facets 176 and 178 are coated withanti-reflective material, current flowing between electrode 174 and 172causes optoelectronic device 100 to operate as an optical gain mediumand generate light that is emitted through the facets. In anotherembodiment of optoelectronic device 100 in which facets 176 and 178 arecoated with anti-reflective material, a voltage applied betweenelectrode 172 and 174 causes optoelectronic device 100 to operate as anoptoelectronic modulator with respect to light passing through theoptical waveguide of which optical waveguide core mesa 140 forms part.

In optoelectronic device 100, n-type cladding layer 120, p-type claddinglayer 160 and growth mask 130 form a capacitor that is a majorcontributor to the inter-electrode capacitance of optoelectronic device100, i.e., the capacitance between electrodes 172 and 174. The growthmask constitutes the dielectric of the capacitor. Mask stripes 132 asthin as about 50 nm will reliably cover growth surface 122, and willtherefore operate effectively as growth mask 130 in the micro-selectivearea growth process described above. However, using a thin growth maskcan result in a high inter-electrode capacitance that limits the maximummodulation speed of the optoelectronic device. In the example ofoptoelectronic device 100 shown, growth mask 130 has a thickness greaterthan the minimum thickness required to cover growth surface 122reliably. A thickness similar to that of optical waveguide core mesa140, i.e., a thickness of about (500±˜150) nm, reduces theinter-electrode capacitance of optoelectronic device 100 to a levelcomparable with that of a conventional buried heterostructureoptoelectronic device with a 3 μm-thick InP:Fe cap layer. Such devicescan be modulated at modulation speeds greater than 10 Gb/s.

Part of p-type cladding layer 160 abuts n-type buffer layer 151 andn-type hole blocking layer 152 (FIG. 4B). However, this arrangement doesnot divert current away from quantum well structure 154 because theturn-on voltage the p-n junction between the p-type cladding layer andthe n-type portions of optical waveguide mesa 140 type is greater thanthat of the p-i-n junction that includes the quantum well structure.

Fabrication of optoelectronic device 100 shown in FIG. 4A will now bedescribed in more detail with reference once again to FIGS. 3A–3H.

Referring first to FIG. 3A, substrate 112 is part of a wafer 110 (FIG.3D) of n-type InP a few hundred micrometers thick. The material of thesubstrate is typically doped with sulfur (S). The crystallineorientation on the major surface 114 of the substrate is [100].

Whereas the material of substrate 112 is nominally the same as that ofn-type cladding layer 120 (FIG. 3B), its crystalline quality and purityis typically below that needed in the cladding layer of anoptoelectronic device. Accordingly, wafer 110 is mounted on thesusceptor in an MOCVD growth chamber and n-type cladding layer 120 isepitaxially grown on the major surface 114 of substrate 112 at a growthtemperature of about 640° C. N-type cladding layer is a layer of InPdoped n-type with silicon. Typical precursors used to grow the n-typecladding layer are trimethylindium ((CH₃)₃In) and phosphine (PH₃) withdisilane (Si₂H₆) as the silicon precursor. Growth continues until n-typecladding layer 120 reaches a thickness of about 2 μm. The surface ofn-type cladding layer provides growth surface 122 whose crystallineorientation is [100].

If InP wafers with a crystalline quality and purity comparable with thatof an epitaxially-grown layer are available, substrate 112 can serve asthe n-type cladding layer of the optoelectronic device. In this case, non-type cladding layer need be epitaxially grown on the substrate, andmajor surface 114 of the substrates provides the growth surface 122 onwhich optical waveguide core mesa 140 is grown by micro-selective areagrowth.

A mask layer (not shown as such) is then deposited on growth surface122. In the example shown, the mask layer is deposited on the surface ofn-type cladding layer 120. In an embodiment, the material of the masklayer was silicon dioxide (SiO₂) formed using silane and oxygen asprecursors. As noted above, the mask layer is typically deposited with athickness of several hundred nanometers to reduce the inter-electrodecapacitance of the optoelectronic device. This thickness is considerablythicker than the minimum thickness needed to reliably cover growthsurface 122. In an embodiment, the mask layer had a thickness of 500 nm.

Wafer 110 is then removed from the growth chamber, and is subject tophotolithography and etching to pattern the mask layer to define themask stripes 132 shown in FIGS. 3C and 3D. Adjacent ones of the maskstripes collectively constitute a growth mask 130 that defines elongategrowth window 134 on growth surface 122. Adjacent ones of the maskstripes 132 are separated by a distance, and, hence, growth window 134has a width, in the range from about 1 μm and about 3 μm, and typicallyin the range from about 1.5 μm–2 μm.

Mask stripes 132 have a width in the range from about 3 μm to about 11μm, and typically in the range from about 5 μm to about 11 μm. Sincesemiconductor material that alights on growth mask 130 migrates towardsthe top surface 146 of optical waveguide core mesa 140 (FIG. 3E) growingin growth window 134, mask stripes that are too wide result in a growthrate that is so fast that the thickness of the grown material isdifficult to control. A high rate of growth is especially problematicaltowards the end of the process of growing the optical waveguide coremesa because the top surface 146 of the mesa is small in area.Consequently, so the growth rate is large and accelerating. On the otherhand, mask stripes 132 that are too narrow allow the p-type claddinglayers 160 grown on the optical waveguide core mesas in adjacent growthwindows 134 to merge, which is also undesirable.

Once growth masks 130 have been formed, wafer 110 is returned to thegrowth chamber. The wafer is heated to a growth temperature of about640° C. and optical waveguide core mesa 140 is grown by micro-selectivearea growth in growth window 134 defined on growth surface 122 by growthmask 130. Growth of an embodiment of optical waveguide core mesa 140having the layer structure shown in FIG. 4B will be described.

N-type buffer layer 151 is a layer of InP grown on growth surface 122.In the example shown, growth surface 122 is the surface of n-typecladding layer 120. The n-type buffer layer is doped n-type withsilicon. Typical precursors used to grow the n-type buffer layer aretrimethylindium ((CH₃)₃In) and phosphine (PH₃) with disilane (Si₂H₆) asthe silicon precursor. Growth continues until n-type buffer layer 151reaches a thickness of about 100 nm.

N-type buffer layer 151 is made as thin as possible to reduce the totalthickness of optical waveguide core mesa 140. Reducing the thickness ofmesa 140 reduces the maximum growth rate of the mesa, i.e., the growthrate during deposition of sublayer 162 of p-type cladding layer 160 whenthe area of the top surface 146 of the mesa is a minimum. It isdesirable to reduce the maximum growth rate to enhance control of thelayer thickness and to maximize the crystalline quality of the materialgrown. Material that grows at a high growth rate can be of lowcrystalline quality because of strain.

In an embodiment in which the process of defining growth mask 130 in alayer of mask material does not degrade the crystalline quality ofgrowth surface 122 and leaves no residue of the mask material in growthwindow 134, n-type buffer layer 151 need not be grown. In this case,hole blocking layer 152 is grown directly on growth surface 122.Omitting n-type buffer layer 151 desirably reduces the thickness ofoptical waveguide core mesa 140.

The supply of phosphine to the growth chamber is cut off, and suppliesof trimethylaluminum ((CH₃)₃Al) and arsine (AsH₃) are commenced to growhole blocking layer 152 of AlInAs on n-type buffer layer 151 (or ongrowth surface 122, as noted above). The precursor flow rates areadjusted to produce the AlInAs with an aluminum fraction that provideslattice matching between the hole blocking layer and the InP of n-typebuffer layer 151. Growth continues until hole blocking layer 152 reachesa thickness of about 40 nm.

A supply of trimethylgallium ((CH₃)₃Ga) is commenced to growsubstrate-side separation layer 153 of AlGaInAs on hole blocking layer152. The precursor flow rates are adjusted to produce the AlGaInAs withAl, Ga and In fractions of 0.325, 0.175 and 0.5, respectively. Materialof this composition is lattice matched to InP. No dopant is added to thematerial of the substrate-side separation layer during growth. Growthcontinues until substrate-side separation layer 153 reaches a thicknessof about 50 nm.

Quantum well structure 154 is grown next. The precursor flow rates arefirst adjusted to grow a barrier layer 158 on substrate-side separationlayer 153. Barrier layer 158 is a layer of AlGaInAs with Al, Ga and Infractions of 0.32, 0.22 and 0.46, respectively. Material of thiscomposition has a band gap energy similar to that of the AlGaInAs ofsubstrate-side separation layer 153 but has a different lattice constantso that the barrier layer is strained. No dopant is added to thematerial of the quantum well structure during growth. Growth continuesuntil barrier layer 158 reaches a thickness of about 8 nm.

The precursor flow rates are then adjusted to grow a quantum well layer157 on barrier layer 158. Quantum well layer 157 is a layer of AlGaInAswith Al, Ga and In fractions of 0.18, 0.22 and 0.6, respectively.Material with this composition has a band gap energy substantially lowerthan that of barrier layer 158. Such material also has a latticeconstant that differs from that of the AlGaInAs of the substrate-sideseparation layer in the opposite direction from that of barrier layer158 so that the quantum well layer is also strained. Growth continuesuntil quantum well layer 157 reaches a thickness of about 9 nm.

The process, similar to that just described, of growing a barrier layer158 followed by a quantum well layer 157 is repeated six times to grow atotal of seven barrier layers and seven quantum well layers. Theprocess, similar to that just described, of growing a barrier layer 158is performed once to grow an eighth barrier layer. This completes growthof quantum well structure 154.

The precursor flow rates are then adjusted to grow remote-sideseparation layer 155 on quantum well structure 154. The precursor flowrates are adjusted to produce AlGaInAs with Al, Ga and In fractions of0.325, 0.175 and 0.5, respectively. Material of this composition islattice matched with InP, but is mis-matched with barrier layer 158. Nodopant is added to the material of the remote-side separation layerduring growth. Growth continues until remote-side separation layer 155reaches a thickness of about 50 nm.

The supply of trimethylgallium to the growth chamber is cut off, and asupply of dimethylzinc ((CH₃)₂Zn) is commenced to grow electron blockinglayer 156 of p-type AlInAs on remote-side separation layer 155, as shownin FIG. 3E. The precursor flow rates are adjusted to produce the AlInAswith an aluminum fraction that provides lattice matching to InP Growthcontinues until electron blocking layer 156 reaches a thickness of about40 nm. This completes fabrication of optical waveguide core mesa 140.

The supplies of trimethylaluminum and arsine (AsH₃) to the growthchamber are cut off, a supply of phosphine (PH₃) is commenced, and thegrowth temperature is reduced to about 620° C. to grow sublayer 162 ofp-type cladding layer 160, as shown in FIG. 3F. Sublayer 162 is a thinlayer of p-type InP and is grown on electron blocking layer 156 bymicro-selective area growth. Sublayer 162 grows on the top surface 146of optical waveguide core mesa 140.

The reduced growth temperature used to grow sublayer 162 of p-typecladding layer is still above the temperature at which the adatoms ofthe semiconductor material have sufficient mobility that their surfacediffusion length is greater than the width w (FIG. 3H) of the [111]surfaces constituting sidewalls 144. Thus, sublayer 162 growspredominantly on the top surface of optical waveguide core mesa 140 asjust described. The growth temperature of about 620° C. can be usedbecause the material being grown lacks aluminum. When growing materialswithout aluminum, there is therefore is no need to use a growthtemperature of about 640° C., which is needed when growingaluminum-containing materials to prevent aluminum from sticking togrowth mask 130. The growth temperature of 620° C. is intermediatebetween the growth temperature used to grow optical waveguide core mesa140 and the growth temperature used below to grow the remainder ofp-type cladding layer 160. Growth of sublayer 162 continues until itreaches a thickness of a few tens of nanometers. In an embodiment,growth continued until sublayer 162 reached a thickness of about 40 nm.

The growth temperature is then reduced to about 600° C. As thetemperature falls, the adatoms of semiconductor material reduce inmobility so that their surface diffusion length becomes less than thewidth of the [111] surfaces constituting sidewalls 144. At the reducedgrowth temperature, micro-selective area growth continues but growth nolonger occurs predominantly on the top surface 140 of optical waveguidecore mesa 140. Instead, p-type cladding layer 160 additionally grows onthe sidewalls 144 of the mesa. Growth of p-type cladding layer 160 atthe reduced growth temperature continues until the p-type cladding layerreaches its specified thickness, as shown in FIG. 3G. In an embodiment,growth continued until p-type cladding layer 160 reached a thickness of2 μm.

After the growth of p-type cladding layer 160 has been completed, wafer110 is removed from the growth chamber. P-type cladding layer 160 coversthe sidewalls 144 of optical waveguide core mesa 140 and extends overpart of growth mask 130. Accordingly, the p-type cladding layer protectssidewalls 144 from damage during subsequent processing applied to thewafer. This processing includes deposition and patterning of electrodes172 and 174, cleaving to form facets 176 and 178 and singulation intoindividual optoelectronic devices.

The invention is described above with reference to an example in whichsome of the semiconductor materials used to fabricate optical waveguidecore mesa 140 comprise aluminum. However, this is not critical to theinvention. None of the semiconductor materials used to fabricate theoptical waveguide core mesa need comprise aluminum. For example, quantumwell structure 154 may have quantum well layers 157 of InGaAsP. Lasershaving such quantum well structures have a lower T₀ than those describedabove in which quantum well structure 154 has quantum well layers 157 ofAlGaInAs.

The invention is described above with reference to examples in whichoptical waveguide core mesa 140 is structured to provide the activeregion of an optoelectronic device. However, embodiments of theinvention are not limited to optoelectronic devices and theirfabrication. The invention additionally encompasses transparentwaveguide devices and their fabrication. Such embodiments of theinvention provide transparent optical waveguides in which opticalwaveguide core mesa 140 has a trapezoidal cross-sectional shape butlacks the layer structure shown in FIG. 4B instead, the opticalwaveguide core mesa is structured as a homogeneous mesa of asemiconductor material having a refractive index higher than that ofcladding layers 120 and 160. Examples of suitable semiconductormaterials include AtinAs, AlGaInAs and InGaAsP.

The invention is described above with reference to examples in which theopposed edges of growth mask 130 are described as being aligned parallelto the [011] crystalline direction of growth surface 122. However,although optimum results are obtained with this alignment of the growthmask, micro-selective area growth is not critically dependent onalignment, and results that are acceptable for use in many applicationsare obtained notwithstanding a deviation from the parallel relationshipdescribed.

FIGS. 5A and 5B are graphs showing some performance characteristics ofan embodiment of an optoelectronic device in accordance with theinvention configured as a buried heterostructure laser. The lasergenerated light at a wavelength of 1350 nm and had a cavity length,i.e., distance between facets 176 and 178 (FIG. 4A), of 300 μm. Thematerial of the quantum well structure was AlGaInAs.

FIG. 5A shows the variation of optical output power and forward voltagedrop with current between electrodes 174 and 172 (FIG. 4A) at tendifferent temperatures, 0° C. through 90° C. in ten-degree intervals.The laser had a threshold current of less than 4 mA at 0° C. and lessthan 20 mA at a temperature of 90° C. The 0° C. threshold current isabout 30% less than a conventional (FIG. 1A–1C) InGaAsP BH laser.

FIG. 5B shows the variation of c-w threshold current and differentialquantum efficiency with temperature. The variation of both thresholdcurrent and efficiency with temperature is relatively small due to thehigher characteristic temperatures obtained using AlGaInAs as thematerial of the quantum well structure. Sample lasers fabricated inaccordance with the invention using AlGaInAs as the material of thequantum well structure had characteristic temperatures of T₀ (thresholdcurrent)=55° K and T₁ (efficiency)=190° K. An otherwise similar lasersfabricated in accordance with the invention using InGaAsP as thematerial of the quantum well structure had characteristic temperaturesof T₀=45° K and T₁=145° K.

This disclosure describes the invention in detail using illustrativeembodiments. However, the invention defined by the appended claims isnot limited to the precise embodiments described.

1. A device, comprising: a growth surface; a growth mask on the growthsurface, the growth mask defining an elongate growth window; an opticalwaveguide core mesa located in the growth window and having atrapezoidal cross-sectional shape; a sublayer formed on a surface of theoptical waveguide core mesa that opposes the growth surface; a claddinglayer covering the sublayer and sidewalls of the optical waveguide coremesa and extending over at least part of the growth mask, the claddinglayer comprising a plane major surface that is substantially parallel tothe growth surface and that extends over at least a part of the growthmask on each side of the growth window; and an electrode formed on theplane major surface of the cladding layer.
 2. The device of claim 1, inwhich: the growth surface has a [100] crystalline orientation; and theoptical waveguide core mesa comprises sidewalls having a [111]crystalline orientation.
 3. The device of claim 2, in which the growthmask comprises opposed edges aligned parallel to the [011] crystallinedirection of the growth surface.
 4. The device of claim 1, in which theoptical waveguide core mesa is homogeneous in structure and has agreater refractive index than the cladding layer.
 5. The device of claim1, in which: the device is an optoelectronic device; and the opticalwaveguide core mesa comprises a quantum well structure.
 6. The device ofclaim 5, in which the quantum well structure comprises quantum welllayers comprising aluminum, gallium, iridium and arsenic.
 7. The deviceof claim 5, in which the quantum well structure comprises quantum welllayers comprising gallium, indium, arsenic and phosphorus.
 8. The deviceof claim 5, in which the optical waveguide core additionally comprises aseparate confinement heterostructure in which the quantum well structureis located.
 9. The device of claim 5, in which the optical waveguidecore mesa comprises materials having a greater refractive index than thecladding layer.
 10. The device of claim 1, in which: the cladding layeris a first cladding layer; the device additionally comprises a secondcladding layer; and the growth surface is a surface of the secondcladding layer.
 11. The device of claim i, in which the growth mask andthe optical waveguide core mesa are similar in thickness.